Interferometric synthetic-aperture radar (InSAR)

Geodesists are, for the most part, a patient and hardworking lot. A day spent hiking to a distant peak, hours spent waiting for clouds to clear a lineof-sight between observation points, weeks spent moving methodically along a level line – such is the normal pulse of the geodetic profession. The fruits of such labors are all the more precious because they are so scarce. A good day spent with an electronic distancemeter (EDM) or level typically produces fewer than a dozen data points. A year of tiltmeter output sampled at ten-minute intervals constitutes less than half a megabyte of data. All of the leveling data ever collected at Yellowstone Caldera fit comfortably on a single PC diskette! These quantities are trivial by modern datastorage standards, in spite of the considerable efforts expended to produce them. Armed with a few hard-won data points, the geodesist must hope that they accurately characterize essential features of the deformation field, without introducing artifacts caused by sparse network coverage or bench mark instability. This is a problem at many volcanoes, where geodetic networks tend to be sparse relative to the complexity of the deformation field. Owing to difficult logistics, safety concerns, or limited resources, many networks are neither dense enough in proximal areas to show the details of shallow-seated deformation nor extensive enough distally to capture deformation from deeper sources (Chapter 11). What a boon it would be to train a magical geodetic camera on a deforming volcano and take a picture of the entire deformation field, rather than trying to piece it together bench mark by bench mark! Imagine a technique capable of producing a detailed snapshot of the deformation field with centimeter accuracy, over lateral dimensions of tens of kilometers, without a requirement for ground access to the field area, even at night or in bad weather. Too good to be true? Not necessarily, at least under favorable conditions that exist at many of the world’s volcanoes. In fact, remotely sensed, remarkably detailed images of volcano deformation started appearing in some of the world’s most prestigious research journals in the mid-1990s, creating a buzz among normally stodgy geodesists – and for good reason. Under ideal conditions, radar images acquired by satellites can provide more information about a deforming volcano than even the most intensive ground-based geodetic surveys. For a geodesist, this is remote sensing at its very best! For example, the radar interferogram in Figure 5.1 reveals that a shallow-dipping dike intruded the southwest flank of Fernandina Volcano in the Galápagos Islands and fed a lava flow that reached the sea – a conclusion confirmed by the accompanying SPOT satellite image. Closer examination and modeling reveal that the maximum surface uplift